Blue laser system for measuring nitrogen dioxide concentration in gaseous mixtures

ABSTRACT

A method of measuring a concentration of NO 2  in a gaseous mixture using a multimode laser beam that covers a tunable spectral range with a width of no more than 5 nm, wherein the multimode laser beam provides a high resolution transmittance spectrum at an absorption cross section of NO 2  molecules, and a system for measuring the concentration of NO 2  in the gaseous mixture. Various combinations of embodiments of the system and the method are provided.

STATEMENT OF FUNDING ACKNOWLEDGEMENT

The funding support provided by King Abdulaziz City for Science andTechnology (KACST) through the Science & Technology Unit at King FandUniversity of Petroleum & Minerals (KFUPM) for funding this work underproject No. 12-ENV2365-04, and Internal Research Grants of King FandUniversity of Petroleum and Minerals under project No. RG1218-1,RG1218-2, RG1422-1, and RG1422-2 are gratefully acknowledged.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINTINVENTOR

Aljalal et al., Detection of trace amount of NO ₂ gas using tunable bluelaser diode. Proceedings of the SPIE, Volume 10231, 102311E, May 162017, which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method of measuring a concentrationof NO₂ in a gaseous mixture using a multimode laser beam, wherein themultimode laser beam provides a high resolution transmittance spectrumat an absorption cross section of NO₂ molecules.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Nitrogen dioxide (NO₂) is one of the air pollutants that are releasedfrom the combustion of fossil fuel, for example, in car engines or powerplants. NO₂ is a reactive compound that has a significant influence onthe chemistry of the atmosphere [Richter, A., Burrows, J. P., Nüß, H.,Granier, C., Niemeier, U., “Increase in tropospheric nitrogen dioxideover China observed from space,” Nature 437(618), 129-132 (2005); J. E.Andrews, P. B., T. D. Jickells, P. S. L. and B. R., An Introduction toEnvironmental Chemistry, Wiley-Blackwell (2004)]. Photochemicalreactions involving NO₂ generally lead to the formation of harmfulchemical compounds in the atmosphere, thus adversely affecting humanlife and the environment, for example, causing acid rain. In addition,NO₂ can react with compounds present in the atmosphere and formcarcinogenic compounds and particles that may penetrate sensitive humanorgans thus causing respiratory diseases [United States EnvironmentalProtection Agency., “Nitrogen Dioxide (NO₂) Pollution,” 2017].

Two safe exposure limits of NO₂ concentration in ambient air are definedby regulatory environmental agencies. The first limit is referred to asa long-term exposure limit, which is an average NO₂ concentration over a1-year period, and the second limit is a short-term exposure limit,which is an average NO₂ concentration over a 1-hour period. Forinstance, the USA Environmental Protection Agency (EPA) has set thelong-term exposure limit to 53 ppb and the short-term exposure limit to100 ppb [United States Environmental Protection Agency., “NitrogenDioxide (NO₂) Pollution,” 2017], and the European Commission Environmenthas set the long-term exposure limit of 20 ppb [European Commission.,“Air Quality Standards,” 2016].

Different instruments have been developed for measuring and monitoringthe concentration of NO₂. These instruments are generally classified asi) point-monitoring instruments that are capable of measuring atspecific locations, and ii) long-path monitoring instruments that arecapable of remote measurement of NO₂ concentrations over extended paths.Point-monitoring instruments generally operate based onchemiluminescence, fluorescence, colorimetry, diffusion,electrochemistry, or electrical resistivity [Liberti, A., “Modem methodsfor air pollution monitoring,” Pure Appl. Chem. 44, 519-534 (1975)].However, long-path monitoring instruments generally operate based on theabsorption cross section of NO₂ molecules in the spectral region in thewavelength range of 300-600 nm. Differential Optical AbsorptionSpectrometers (DOAS), Tunable Diode Laser Absorption Spectrometers(TDLAS), and Differential Absorption LIDAR devices (DIAL) are amongthese types of instruments [Platt, U., Stutz, J., Differential OpticalAbsorption Spectroscopy Principles and Applications, Springer (2008);Kormann, R., Fischer, H., Gurk, C., Helleis, F., Klu, T., Kowalski, K.,Parchatka, U., Wagner, V., “Application of a multi-laser tunable diodelaser absorption spectrometer for atmospheric trace gas measurements atsub-ppbv levels,” Spectrochim. Acta Part A 58, 2489-2498 (2002); Menyuk,N., Killinger D. K., DeFeo, W. E., “Remote sensing of NO using adifferential absorption lidar,” Appl. Opt. 19(19), 3282-3286 (1980)].Although, DOAS appear to be the most widely used instruments amonglong-path monitoring instruments [United States Environmental ProtectionAgency., “List of designated reference and equivalent methods” (2016)],the availability of blue diode lasers makes TDLAS an attractivealternative to DOAS. In contrast to DOAS, TDLAS can generate light beamswith higher spectral intensities and greater collimation, thus creatinglarger signal-to-noise ratios without the need for expensive collimationoptical components.

Blue laser diodes have been used to detect NO₂ using laser inducedfluorescence [Parra, J., George, L. A., Parra, J., George, L. A.,“Development of an ambient pressure laser-induced fluorescenceinstrument for nitrogen dioxide Development of an ambient pressurelaser-induced fluorescence instrument for nitrogen dioxide,” Appl. Opt.48(18), 3355-3361 (2009); Taketani, F., Kawai, M., Takahashi, K.,Matsumi, Y., “Trace detection of atmospheric NO₂ by laser-inducedfluorescence using a GaN diode laser and a diode-pumped YAG laser,”Appl. Opt. 46(2), 907-915 (2007)], photoacoustic detection [Yi, H., Liu,K., Chen, W., Tan, T., Wang, L., Gao, X., “Application of a broadbandblue laser diode to trace NO₂ detection using off-beam quartz-enhancedphotoacoustic spectroscopy,” Opt. Lett. 36(4), 481-483 (2011); Kalkman,J., Kesteren, H. Van., “Relaxation effects and high sensitivityphotoacoustic detection of NO₂ with a blue,” Appl. Opt. 200(2), 197-200(2008)], cavity-enhanced absorption spectroscopy [Courtillot, I.,Morville, J., Motto-ros, V., Romanini, D., “Sub-ppb NO₂ detection byoptical feedback cavity-enhanced absorption spectroscopy with a bluediode laser,” 407-412 (2006); Wojtas, J., Mikolajczyk, J., Bielecki, Z.,“Aspects of the Application of Cavity Enhanced Spectroscopy to NitrogenOxides Detection,” Sensors 13(x), 7570-7598 (2013)], and single-passabsorption [Liu., J. T. C., Hanson, R. K., Jeffries, J. B.,“High-sensitivity absorption diagnostic for NO₂ using a blue diodelaser,” J. Quant. Spectrosc. Radiat. Transf. 72(2), 655-664 (2002);Yang, Y., Gao, Z., Zhong, D., Lin, W., “Detection of nitrogen dioxideusing an external modulation diode laser,” Appl. Opt. 52(2), 2-5(2013)].

Employing blue laser diodes provides detection of NO₂ in open-air pathsat very low detection limits. However, in the blue region, the sharpfeatures of the NO₂ absorption spectrum are relatively broad with anaverage width of a few nanometers. Therefore, detection andconcentration measurement of NO₂ becomes very challenging whenconventional methods of direct or wave-modulated absorption spectroscopyis employed.

In view of the forgoing, one objective of the present disclosure is toprovide a method of measuring a concentration of NO₂ in a gaseousmixture using a multimode laser beam that covers a tunable spectralrange with a width of no more than 5 nm, wherein the multimode laserbeam provides a high resolution transmittance spectrum at an absorptioncross section of NO₂ molecules.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to a methodof measuring a concentration of nitrogen dioxide in a gaseous mixture,involving i) generating a multimode laser beam with a tunable laserdiode at an injection current and a diode temperature, ii) splitting themultimode laser beam into a reference beam and a signal beam, iii)passing the signal beam through the gaseous mixture and recording afirst voltage signal with a first detector, iv) passing the referencebeam through a reference medium and recording a second voltage signalwith a second detector, v) measuring a transmittance of the signal beamat the injection current, wherein the transmittance is a ratio of thefirst voltage signal to the second voltage signal, vi) tuning themultimode laser beam by varying the injection current and repeating thesplitting, passing the signal beam, passing the reference beam, and themeasuring to form a high resolution transmittance spectrum, vii)measuring a column density of nitrogen dioxide in the gaseous mixturefrom the high resolution transmittance spectrum, viii) calculating theconcentration of nitrogen dioxide in the gaseous mixture from the columndensity.

In one embodiment, the multimode laser beam has a wavelength in therange of 250-650 nm.

In one embodiment, the multimode laser beam is a blue laser beam with awavelength in the range of 445-455 nm.

In one embodiment, the multimode laser beam covers a tunable spectralrange with a width of no more than 5 nm.

In one embodiment, the tuning is carried out by increasing the injectioncurrent from 30 mA to 160 mA with an increment of 0.05-0.2 mA.

In one embodiment, the multimode laser beam is tuned at an injectioncurrent resolution of 0.002-0.008 nm/mA.

In one embodiment, the tuning is carried out in a wavelength region witha width of no more than 5 nm.

In one embodiment, the diode temperature is set to a value in the rangeof 10−60° C. before the splitting.

In one embodiment, the transmittance of the signal beam is measured in awavelength region of 445-450 nm.

In one embodiment, the transmittance of the signal beam is measured in awavelength region of 447-449 nm.

In one embodiment, the gaseous mixture is air, wherein the signal beamis passed through the air with a beam path length of no more than 600 m.

In one embodiment, the reference medium is air, wherein the referencebeam is passed through the air with a beam path length of no more than5.0 m.

In one embodiment, the reference medium is an NO₂-free gaseous mixture,wherein the reference beam is passed through the NO₂-free gaseousmixture with a beam path length of no more than 100 m.

In one embodiment, a detection limit for the column density of nitrogendioxide in the gaseous mixture ranges from 0.1-1.0 ppm.m.

In one embodiment, the signal beam is passed through the gaseous mixturewith a beam path length of 100-600 m, wherein a detection limit for theconcentration of nitrogen dioxide in the gaseous mixture ranges from0.5-10 ppb.

In one embodiment, the gaseous mixture consists of nitrogen dioxide andnitrogen gas and the reference medium consists of nitrogen gas.

In one embodiment, the gaseous mixture is disposed in an elongatedcompartment with a length of 0.01-10 m, wherein at least a portion ofthe elongated compartment is transparent to a UV light and/or a visiblelight.

In one embodiment, the gaseous mixture is present in the elongatedcompartment at a pressure of 0.5-1.5 atm.

In one embodiment, the elongated compartment has a length of 0.1-2.0 m,wherein a detection limit for the concentration of nitrogen dioxide inthe gaseous mixture ranges from 0.1-10 ppm.

According to a second aspect, the present disclosure relates to a systemfor measuring a concentration of nitrogen dioxide in a gaseous mixture,the system including i) an elongated compartment with a first end and asecond end separated by a side wall along a longitudinal axis of theelongated compartment, wherein at least a portion of the first end andat least a portion of the second end is transparent to a UV light and/ora visible light, ii) at least one sealable aperture arranged on theelongated compartment for delivering a gaseous mixture to the elongatedcompartment and/or discharging the gaseous mixture from the elongatedcompartment, iii) a tunable laser diode located at a distance from theelongated compartment for generating a multimode laser beam and emittingthe multimode laser beam to the elongated compartment such that themultimode laser beam is substantially parallel to the longitudinal axisof the elongated compartment, iv) a laser diode current controller thatis electrically connected to the tunable laser diode for varying aninjection current of the tunable laser diode, v) a laser diodetemperature controller that is electrically connected to the tunablelaser diode for varying a diode temperature of the tunable laser diode,vi) a beam splitter located between the tunable laser diode and theelongated compartment for splitting the multimode laser beam into asignal beam and a reference beam, wherein the signal beam is passedthrough the gaseous mixture inside the elongated compartment and thereference beam is passed through a reference medium, vii) a firstdetector for detecting the signal beam after being passed through thegaseous mixture inside the elongated compartment, viii) a seconddetector for detecting the reference beam after being passed through thereference medium, ix) a computer connected to the first and the seconddetectors, wherein the computer receives a first voltage signal from thefirst detector and a second voltage signal from the second detector andcalculates the concentration of nitrogen dioxide in the gaseous mixture.

In one embodiment, the tunable laser diode is a multimode blue laserdiode that generates a blue laser beam with a wavelength in the range of445-455 nm.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a system for measuring a concentrationof nitrogen dioxide in a gaseous mixture.

FIG. 2 represents a high resolution absorption spectrum of a laser beamat an absorption cross-section of NO₂ in the wavelength region of446-450 nm, wherein (a) is the absorption cross section of NO₂, and thelaser beam is generated using (b) a tunable laser diode with aninjection current of 30 mA, (c) a tunable laser diode with an injectioncurrent of 100 mA, (d) a tunable laser diode with an injection currentof 160 mA.

FIG. 3 represents a numerical and an experimental transmittance spectrumof a multimode laser beam through a gaseous mixture, wherein themultimode laser beam is tuned by varying the injection current at atemperature of 40° C.

FIG. 4 represents a numerical and an experimental transmittance spectrumof a multimode laser beam through a gaseous mixture, wherein themultimode laser beam is tuned by varying the injection current at atemperature of 50° C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure will be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of “one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. Also, all values and subrangeswithin a numerical limit or range are specifically included as ifexplicitly written out.

As used in this disclosure, the term “substantially the same” refers toan embodiment or embodiments where a difference between two quantitiesare no more than 2%, preferably no more than 1%, preferably no more than0.5% of the smaller value of the two quantities.

According to a first aspect, the present disclosure relates to a methodof measuring a concentration of nitrogen dioxide (NO₂) in a gaseousmixture.

The method involves generating a multimode laser beam with a tunablelaser diode (LD). In terms of the present disclosure, the “tunable laserdiode” refers to a device having a gain medium of a doped semiconductormaterial as a p-n junction or a p-i-n junction, wherein a gain isgenerated by an electrical current flowing through the p-n junction orthe p-i-n junction. Accordingly, electrons and holes can recombinethereby releasing energy in the form of light photons. In someembodiments, the tunable laser diode may be a semiconductor laser. Themultimode laser beam may be generated using other types of lasers, forexample, quantum cascade lasers or optically pumped semiconductorlasers. In certain embodiments, the tunable laser diode may be anoptical fiber laser with a gain medium comprising an optical fiber dopedwith one or more rare-earth elements selected from the group consistingof erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium.In some embodiments, the tunable laser diode may be a quantum-dot typesemiconductor laser as known to those skilled in the art. A quantum-dottype semiconductor laser is a type of semiconductor laser that employs alayer of quantum dots as an active gain medium in a light emittingregion. Due to a tight confinement of charge carriers in threedimensions, quantum dots may exhibit an electronic structure similar toatoms where energy levels can be adjusted by controlling quantum dotdimensions or quantum dot material compositions. An exemplaryquantum-dot type semiconductor laser may be composed of indium galliumarsenide and/or gallium arsenide. The quantum-dot type semiconductorlaser may further include an external cavity holographic grating thatmay stabilize a wavelength of a generated laser beam.

In some embodiments, the tunable laser diode may be selected from one ormore laser diodes selected from small edge-emitting LDs, distributedfeedback lasers (or DFB lasers), distributed Bragg reflector lasers (DBRlasers), external cavity diode lasers, broad area laser diodes (mayalternatively be referred to as broad stripe laser diodes, wide stripelaser diodes, or high brightness laser diodes), slab-coupled opticalwaveguide lasers (SCOWLs), high-power diode bars, high-power stackeddiode bars, and monolithic surface-emitting semiconductor lasers(VCSELs). In a preferred embodiment, the tunable laser diode comprisesindium gallium nitride and/or gallium nitride on a silicon carbidesubstrate. The tunable laser diode may emit the multimode laser beaminto a free space, or it may transmit the multimode laser beam to anoptical fiber.

In a preferred embodiment, the tunable laser diode generates a coherentnarrow-bandwidth light emission whose wavelength is tunable across oneor more absorption cross section of NO₂ molecules as stated by Yoshinoet al [Yoshino, K., Esmond, J. R., Parkinson, W. H., “High-resolutionabsorption cross section measurements of NO₂ in the UV and visibleregion,” Chem. Phys. 221(2), 169-174 (1997)—incorporated herein byreference in its entirety]. Accordingly, in some embodiments, thetunable laser diode is a multimode blue laser diode that generates acoherent narrow-bandwidth light emission whose wavelength is tunableacross one or more absorption cross section of NO₂ molecules in awavelength region of 350-500 nm, preferably 400-480 nm, preferably440-450 nm. For example, in some preferred embodiments, the tunablelaser diode is a multimode blue laser diode that generates a blue laserbeam with a wavelength in the range of 445-455 nm, preferably 446-450nm, preferably 447-449 nm. For example, in one embodiment, the tunablelaser diode is a Fabry-Perot blue laser diode. In one embodiment, thetunable laser diode is a commercially available blue laser diodeobtained from Roithner LaserTechnik GmbH model LD-445-50PD with a poweroutput of up to 50 mW.

The multimode laser beam may preferably cover a tunable spectral rangewith a width of no more than 5 nm, preferably 1-4 nm, preferably 2-3 nm.As used in this disclosure, the term “tunable spectral range” refers toa wavelength region over which the multimode laser beam can be tuned,for example by varying an injection current of the tunable laser diode.Accordingly, the multimode laser beam may preferably provide a highresolution absorption or transmittance spectrum of the gaseous mixtureat an absorption cross section of NO₂ molecules in a wavelength regionof interest at a width of no more than 5 nm, preferably 1-4 nm,preferably 2-3 nm. Employing the aforementioned multimode laser beam mayallow sensitive detection of NO₂ in a gaseous mixture at pronouncedabsorption cross sections of NO₂ molecules in the wavelength region250-650 nm, preferably 350-500 nm, as mentioned by Yoshino et al[Yoshino, K., Esmond, J. R., Parkinson, W. H., “High-resolutionabsorption cross section measurements of NO₂ in the UV and visibleregion,” Chem. Phys. 221(2), 169-174 (1997)—incorporated herein byreference in its entirety]. For example, in a preferred embodiment, themultimode laser beam may cover a tunable spectral range of 445-450 nm,preferably 446-449 nm, preferably 447-449 nm. In another embodiment, themultimode laser beam may cover a tunable spectral range of 421-426 nm,preferably 422-425 nm, preferably 423-425 nm. In still anotherembodiment, the multimode laser beam may cover a tunable spectral rangeof 432-438 nm, preferably 433-437 nm. Accordingly, misidentifyingabsorption signatures that may be recorded in the presence ofinterferers, e.g. dust particles, in the gaseous mixture may preferablybe substantially reduced.

The tunable laser diode may have an output power in the range of 10 mWto 5W, preferably 20-1,000 mW, preferably 30-500 mW, preferably 40-200mW, preferably about 50 mW.

In some preferred embodiments, the multimode laser beam is generated atan injection current which is preferably in the range of 30-160 mA,preferably 35-155 mA, preferably 40-150 mA, and a diode temperaturewhich is preferably in the range of 10-60° C., preferably 15-55° C.,preferably 20-50° C., preferably 25-50° C. According to theseembodiments, the multimode laser beam is preferably generated at a fixedtemperature, i.e. 40° C. or 50° C., wherein the injection current isramped up.

In certain embodiments, an optically non-linear crystal is opticallycoupled to the tunable laser diode, for example, using optical fibers,wherein the optically non-linear crystal is configured to non-linearlyvary a frequency of the multimode laser beam. In addition, in oneembodiment, an optical filter may be optically coupled to the opticallynon-linear crystal, wherein the optical filter is configured to onlytransmit light photons with a preferable wavelength range, as mentionedpreviously, while blocking transmission of non-preferable light photons.In certain embodiments, the tunable laser diode may be combined with anexternal optical resonator, which allows a beam of light to circulate ina closed path. In some embodiments, the tunable laser diode may becombined with additional optics as known to those skilled in the art,such as beam collimators or beam shapers, means for coupling the lightto an optical fiber (e.g. fiber-coupled diode lasers), means to provideoptical feedback, etc. In certain embodiments, the multimode laser beamis generated with multiple tunable laser diodes and means for beamcombining such as, e.g. coherent beam combining devices or spectral beamcombining devices as known to those skilled in the art.

Once the multimode laser beam is generated, the method further involvessplitting the multimode laser beam into a reference beam and a signalbeam with a beam splitter.

The beam splitter is an optical device that splits an incident lightbeam (e.g. a laser beam) into two or more beams, which may or may nothave identical intensities. Any type of the beam splitter as known tothose skilled in the art may be utilized here, for example, the beamsplitter may be a dielectric mirror, a beam splitter cube, a fiber-opticbeam splitter, a metal-coated mirror, a pellicle, a micro-optic beamsplitter, a waveguide beam splitter, etc. In one embodiment, the beamsplitter is optically coupled to the multimode laser beam at anincidence angle of 30-60°, preferably 35-55°, preferably 40-50°,preferably about 45°. In another embodiment, the beam splitter is a beamsplitter cube, which may be made of two triangular glass prisms that areglued together with some transparent resin or cement therebetween. Incertain embodiments, crystalline media may be used in lieu of thetriangular glass prisms, which allows construction of polarizing beamsplitter cubes such as, e.g., Wollaston prisms, Nomarski prisms,Glan-Thompson prisms, Nicol prisms, etc. In another embodiment, the beamsplitter is a fiber-optic beam splitter, which is made byfusion-combining of optical fibers. The fiber-optic beam splitter mayhave more than one output ports.

In a preferred embodiment, the multimode laser beam is split such thatthe reference beam and the signal beam are substantially the same.Accordingly, the reference beam and the signal beam have substantiallythe same output powers and substantially the same wavelength ranges.

In some embodiments, the multimode laser beam may be split with asecondary beam splitter before splitting the same with the beamsplitter, wherein said split beam is directed to a spectrometer forvisualizing and calibrating the multimode laser beam. In one embodiment,the spectrometer consists of a monochromator with a resolution of0.01-0.05 nm (e.g. SPEX 500M monochromator) and a Thorlabs LC1-USB linecamera that are connected to a computer.

In some preferred embodiments, the tunable laser diode and the beamsplitter are disposed in a thermally insulated enclosure, which isconfigured to provide temperature stabilization to the tunable laserdiode. In some embodiments, each of the optical components, e.g. thebeam splitter, optically non-linear crystals, optical filters, externaloptical resonators, beam collimators, etc., when present, are disposedin the thermally insulated enclosure. In one embodiment, the thermallyinsulated enclosure is a commercially available laser diode mount modelTCLDM9 provided by Thorlabs.

In one embodiment, the tunable laser diode is electrically connected toa laser diode current controller for varying the injection current inthe range of 30-160 mA, preferably 35-155 mA, preferably 40-150 mA withan increment of 0.05-0.2 mA, preferably 0.1 mA. As used here, the laserdiode current controller is a device that adjusts the injection currentof the tunable laser diode. Increasing the injection current of thetunable laser diode at a fixed diode temperature may preferably increaselongitudinal modes of the multimode laser diode and may also broaden thespectral range of the multimode laser beam. In one embodiment, the laserdiode current controller is a commercially available LDC200C provided byThorlabs. In one embodiment, the laser diode current controller is incommunication with a computer preferably using a LabView and a 16-bitinterface card from National Instrument model USB-6251. In someembodiments, increasing the injection current of the tunable laser diodemay shift the laser spectra to larger wavelength ranges, as shown inFIG. 2. In view of that, the method may preferably involve tuning themultimode laser beam by varying the injection current of the tunablelaser diode in the range of 30-160 mA before splitting the multimodelaser beam. In a preferred embodiment, the tunable laser diode includesgallium nitride, which generates a multimode blue laser diode, whereinthe blue laser diode is tuned at an injection current resolution of0.002-0.008 nm/mA, preferably 0.003-0.007 nm/mA, preferably about 0.005nm/mA. Accordingly, a tuning range of the tunable laser diode by varyingthe injection current is no more than 0.8 nm, preferably no more than0.6 nm. The term “injection current resolution” as used herein is aquantity that determines a sensitivity of tuning with respect to varyingthe injection current. Varying the injection current of the tunablelaser diode may allow fine-tuning the absorption spectrum of themultimode laser beam to the wavelength regions of interest, for example,the cross section of NO₂ molecules.

In one embodiment, the tunable laser diode is electrically connected toa laser diode temperature controller for varying the diode temperaturein the range of 10-60° C., preferably 15-55° C., preferably 20-50° C.,preferably 25-50° C., preferably 30-45° C. In one embodiment, the laserdiode temperature controller is a commercially available TED200Cprovided by Thorlabs. The multimode laser beam is temperature sensitive,and diode temperature may influence thermal population distributions invalence and conduction bands in the gain medium of the tunable laserdiode. In view of that, the diode temperature of the tunable laser diodeis preferably set to a constant value before varying the injectioncurrent. In a preferred embodiment, the tunable laser diode includesgallium nitride, which generates a multimode blue laser diode, whereinthe blue laser diode is tuned at a temperature resolution of 0.01-0.02nm/° C., preferably 0.12-0.18 nm/° C., preferably about 0.15 nm/° C.Accordingly, a tuning range of the tunable laser diode by varying thediode temperature is no more than 2 nm, preferably 1-2 nm. The term“temperature resolution” as used herein is a quantity that determines asensitivity of tuning with respect to varying the diode temperature.Varying the diode temperature of the tunable laser diode may allowtuning the absorption spectrum of the multimode laser beam to thewavelength regions of interest, for example, the cross section of NO₂molecules. In certain embodiments, the laser diode temperaturecontroller includes a thermistor temperature sensor disposed on thetunable laser diode, a thermoelectric cooler element on which thetunable laser diode is mounted, and a micro-controller. In anotherembodiment, the laser diode temperature controller is in communicationwith a computer preferably using a LabView and a 16-bit interface cardfrom National Instrument model USB-6251.

Once the multimode laser beam is generated and properly tuned, themethod further involves passing the signal beam through the gaseousmixture and recording a first voltage signal with a first detector.

As used in this disclosure, the term “gaseous mixture” refers to agaseous composition that includes nitrogen dioxide (NO₂) and one or moreof, without limitation, nitrogen gas, oxygen, carbon dioxide, carbonmonoxide, argon, helium, hydrogen, water vapor, nitrogen oxide, methane,neon, krypton, nitrous oxide, sulfur dioxide, etc. In some embodiments,the gaseous mixture only consists of nitrogen dioxide and nitrogen gas.In some embodiments, in order to detect and measure the concentration ofNO₂ in the gaseous mixture, said concentration is at least 0.5 ppb,preferably at least 1 ppb, preferably at least 5 ppb, preferably atleast 10 ppb, preferably at least 50 ppb, preferably at least 100 ppb,preferably at least 200 ppb, preferably at least 500 ppb, preferably atleast 1 ppm. In one embodiment, the gaseous mixture is disposed in aclosed container (e.g. an elongated compartment) and the signal beam ispassed through the gaseous mixture inside the closed container. In oneembodiment, the gaseous mixture is delivered to the closed container(e.g. the elongated compartment) in a continuous fashion at a volumetricflow rate of 50-200 ml/min, preferably 80-150 ml/min, preferably about100 ml/min. In one embodiment, the gaseous mixture is air and the signalbeam is passed through the air with a beam path length of no more than600 m.

The method further involves passing the reference beam through areference medium and recording a second voltage signal with a seconddetector.

As used in this disclosure, the term “reference medium” refers to agaseous composition, which is used for passing the reference beamtherethrough. The reference medium may or may not contain nitrogendioxide depending on the beam path length of the reference beam. Forexample, in one embodiment, the reference medium is air, which maycontain nitrogen dioxide with a concentration of no more than 100 ppm,preferably no more than 50 ppm, preferably no more than 10 ppm, whereinthe reference beam is passed through the reference medium with a beampath length of no more than 5.0 m, preferably no more than 2.0 m,preferably no more than 1.0 m. In one embodiment, the reference mediumis an NO₂-free gaseous mixture, wherein the reference beam may be passedthrough the NO₂-free gaseous mixture with a beam path length of up to100 m, preferably up to 50 m, preferably up to 10 m. The NO₂-freegaseous mixture as used here may refer to a gaseous composition thatincludes one or more of nitrogen gas, oxygen, carbon dioxide, carbonmonoxide, argon, helium, hydrogen, water vapor, nitrogen oxide, methane,neon, krypton, nitrous oxide, sulfur dioxide, and the like, and does notinclude nitrogen dioxide. The reference medium is preferably of the samecomposition as the gaseous mixture but for the exclusion of NO₂. Thereference medium can be obtained by treating the gaseous mixture toremove NO₂ thereby providing a reference medium that is of substantiallythe same composition as the gaseous mixture but for the exclusion ofNO₂.

The term “beam path length” as used in this disclosure refers to a totaldistance that a laser beam (e.g. the signal beam or the reference beam)travels from a source (e.g. the beam splitter) to a detector (e.g. thefirst or the second detector). In circumstances when a laser beamtravels in a linear pathway, the beam path length is identical to adistance between the source (e.g. the beam splitter) and the detector(e.g. the first or the second detector).

In a preferred embodiment, each of the first and the second detector isa photodetector having a p-n junction that converts light photons, e.g.,laser beam photons into an electric current. Preferably, the firstdetector is substantially the same as the second detectors, wherein eachis a silicon photodiode detector, e.g. Thorlabs model PDB 210A. Thefirst and the second detectors may be in communication with a computerfor recording and visualizing the first and the second voltage signals.Various types of photodetectors as known to those skilled in the art maybe utilized in lieu of silicon photodiode detectors for recording thefirst and the second voltage signals. Exemplary include, withoutlimitation, photoemission or photoelectric detectors, thermalphotodetectors, polarization detectors, photochemical detectors, weakinteraction photodetectors, etc. In certain embodiments, neutral densityfilters may be utilized to attenuate the signal beam and the referencebeam intensities before recording the first and the second voltagesignals. In another preferred embodiment, each of the first and thesecond detector is a photodiode with a p-n junction based on indiumarsenide (InAs), indium antimonide (InSb), or mercury cadmium telluride(MCT) that has an electronic response (either as a photocurrent or avoltage) that is sensitive to a UV/Vis light with a wavelength in therange of 250-650 nm, preferably 350-500 nm. Each of the first and thesecond detector may be provided as a laser module that includes aninternal thermoelectric cooler, an internal temperature sensor such as athermistor, a signal amplifier such as a trans-impedance amplifier thatconverts current into a proportional voltage, a digitizer, etc. Incertain embodiments, the first and the second voltage signals may beamplified to improve a signal-to-noise ratio.

Since the signal beam is passed through the gaseous mixture, thetransmittance of the signal beam may provide information about molecularnumber density C at molecular cross section σ(λ) according to theLambert-Beer's law. The Lambert-Beer's law states that when a light ofintensity I₀(λ) passes through a sample of length L with molecularnumber density C and molecular cross section σ(λ), an intensity oftransmitted light I₁(λ) is given by the following equation:

I _(t)(λ)=I ₀(λ)e ^(−σ(λ)CL)  (1)

where each of the incident light I₀ and the cross section a is afunction of the light wavelength λ. Accordingly, if a light is directlydetected by an optical detector, such as a photodiode, a signal detectedby the optical detector is proportional to an intensity of the lightwhen integrated over the wavelength range of the light.

A transmittance of a light may be measured by two detectors, one formeasuring an incident beam (or the reference beam in terms of thepresent disclosure) and the other for measuring a transmitted beam (orthe signal beam in terms of the present disclosure). Accordingly, themeasured transmittance T is given by the following equation:

$\begin{matrix}{T = \frac{\int_{\lambda\; s}^{\lambda_{{e_{I_{0}}}_{{(\lambda)}e^{{- {\sigma{(\lambda)}}}{CL}_{d\;\lambda}}}}}}{\int_{\lambda_{s}}^{\lambda_{e_{{I_{I_{0}}{(\lambda)}}d\;\lambda}}}}} & (2)\end{matrix}$

where λ_(s) and λ_(e) are start and end points of the spectral range ofthe incident light, respectively.

In view of the abovementioned, if a light has a spectral range with awidth smaller than one or more features of a cross section σ(λ), and ifthe light can be tuned over said features, then a variable transmittancemay be obtained as a function of a tuning parameter of the light.

In the present disclosure, the multimode laser beam may be tuned byvarying the injection current at a fixed diode temperature to produce avariable transmittance as a function of injection current and a signalbeam wavelength, λ. Accordingly, at a fixed diode temperature, thetransmittance T is a function of an injection current and a signal beamwavelength, λ, as shown in the following equation:

$\begin{matrix}{{T(i)} = {\frac{\int_{\lambda_{s}{(i)}}^{\lambda_{e^{(i)}{I_{0}{({\lambda,i})}}e} - {{\sigma{(\lambda)}}{CL}_{d\;\lambda}}}}{\int_{\lambda_{s}{(i)}}^{{\lambda_{e}{(i)}}_{{I_{0}{({\lambda,i})}}d\;\lambda}}}.}} & (3)\end{matrix}$

The shape of the transmittance curve may depend on the diodetemperature. To enhance a signal-to-noise ratio in a transmittancespectrum of the signal beam, the diode temperature may be preferablyfixed at a temperature in the range of 25-60° C., preferably 30-50° C.,preferably 40-50° C., wherein the injection current of the tunable laserdiode is ramped up with an increment of 0.05-0.2 mA, preferably 0.1 mA.FIGS. 3 and 4 represent transmittance spectra of a gaseous mixtureconsisting of nitrogen dioxide and nitrogen gas at various injectioncurrents of the signal beam at a diode temperature of 40° C. and 50° C.,respectively.

In cases of small absorptions, for example, in the case of measuring theconcertation of NO₂ in air, equation (3) can be approximated as:

$\begin{matrix}{{T(i)} \cong {1 - {{CL}{\frac{\int_{\lambda_{s}{(i)}}^{{{\lambda_{e}{(i)}}_{I_{0}}}_{{({\lambda,i})}{\sigma{(\lambda)}}d\;\lambda}}}{\int_{\lambda_{s}{(i)}}^{{\lambda_{e}{(i)}}_{{I_{0}{({\lambda,i})}}{\sigma{(\lambda)}}d\;\lambda}}}.}}}} & (4)\end{matrix}$

The column density CL can be measured by fitting the transmittance tothe right-hand side of either Eq. 3 or Eq. 4 with CL as a fittingparameter. The ratio of integrals in Eq. 4 may be obtainedexperimentally by measuring the transmittance of a well-known columndensity. For example, in one embodiment, the transmittance of a gaseousmixture consisting of nitrogen dioxide and nitrogen gas with a constantNO₂ concentration of 200 ppm is measured in a 50 cm column (i.e. acolumn density CL of 100 ppm.m), and the ratio of integrals in Eq. 4 maybe calculated thereafter. In an alternative embodiment, the ratio ofintegrals in Eq. 4 may be obtained by measuring integrals of themultimode laser beam at a known cross section, σ, which may be obtainedfrom literature at different injection currents.

In view of the present disclosure, to measure the column density CL, thetransmittance of the signal beam needs to be measured at variousinjection currents to produce a high resolution transmittance spectrum,which represents the transmittance of the signal beam through thegaseous mixture as a function of injection current at a fixed diodetemperature, as shown in FIGS. 3 and 4.

Accordingly, in a preferred embodiment, the transmittance of the signalbeam is measured by taking a ratio of the first voltage signal, which isobtained from the first detector, to the second voltage signal, which isobtained from the second detector. The first voltage signal is obtainedfrom the signal beam, which may preferably be passed through air or thegaseous mixture, as described previously. The second voltage signal isobtained from the reference beam, which may preferably be passed throughthe reference medium, as described previously. Accordingly, thetransmittance may be measured using the following equation:

$\begin{matrix}{T = \frac{(V)_{{gaseous}\mspace{14mu}{{mix}.}}}{(V)_{{ref}.\mspace{14mu}{medium}}}} & (5)\end{matrix}$

where V_(sig) and V_(ref) are the voltages of the signal detector andthe reference detector, respectively.

In another preferred embodiment, each of the signal beam and thereference beam is normalized before measuring the transmittance.Accordingly, the transmittance of the signal beam is measured by takinga ratio of a first transmittance to a second transmittance, wherein thefirst transmittance is a ratio of the first voltage signal to the secondvoltage signal when the signal beam is passed through the gaseousmixture, whereas the second transmittance is ratio of the first voltagesignal to the second voltage signal when the signal beam is passedthrough the reference medium. Accordingly, the transmittance of thesignal beam may be measured using the following equation:

$\begin{matrix}{T = \frac{\left( {V_{sig}/V_{ref}} \right)_{{gaseous}\mspace{14mu}{{mix}.}}}{\left( {V_{sig}/V_{ref}} \right)_{{ref}.\mspace{14mu}{medium}}}} & (6)\end{matrix}$

For example, in one embodiment, the gaseous mixture consists of nitrogendioxide and nitrogen gas and the reference medium consists of nitrogengas, wherein the transmittance may be measure using the followingequation:

$\begin{matrix}{T = \frac{\left( {V_{sig}/V_{ref}} \right)_{{NO}_{2}/N_{2}}}{\left( {V_{sig}/V_{ref}} \right)_{N_{2}}}} & (7)\end{matrix}$

Once the transmittance of the signal beam is measured at an individualinjection current, the multimode laser beam is tuned by varying theinjection current of the tunable laser diode. In a preferred embodiment,tuning the multimode laser beam is carried out by increasing theinjection current from 30-160 mA, preferably 40-150 mA, with anincrement of 0.05-0.2 mA, preferably 0.1 mA. In some embodiment, theincrement ranges from 0.5-15 mA, preferably 1-10 mA. In one embodiment,the multimode laser beam is tuned at an injection current resolution of0.002-0.008 nm/mA, preferably 0.003-0.007 nm/mA, preferably about 0.005nm/mA. At each individual injection current, the transmittance of thesignal beam is measured by following the above-mentioned steps, therebyforming a high resolution transmittance spectrum that represents thetransmittance of the signal beam at various injection currents.

Preferably, the multimode laser beam is tuned across one or morepronounced absorption cross sections of NO₂ molecules as stated byYoshino et al [Yoshino, K., Esmond, J. R., Parkinson, W. H.,“High-resolution absorption cross section measurements of NO₂ in the UVand visible region,” Chem. Phys. 221(2), 169-174 (1997)—incorporatedherein by reference in its entirety]. Accordingly, in a preferredembodiment, the multimode laser beam is tuned across a wavelength regionof 445-450 nm, preferably a wavelength region of 446-449 nm, preferablya wavelength region of 447-449 nm, as shown in FIG. 2. In some preferredembodiments, tuning the multimode laser beam is carried out in awavelength region with a width of no more than 5 nm, preferably 1-4 nm,preferably 2-3 nm.

The high resolution transmittance spectrum, which may be a function ofinjection current, is preferably approximated to be proportional to thecolumn density CL and a convolution of an intensity of the multimodelaser beam and cross section of NO₂ as shown in Eq. 4. Since the highresolution transmittance spectrum has different shapes at differentdiode temperatures, as shown in FIGS. 3 and 4, a preferred diodetemperature may be a temperature at which the high resolutiontransmittance spectrum produces a more pronounced curvature with ahigher signal-to-noise ratio. Accordingly, in a preferred embodiment,the diode temperature is set to a value in the range of 10-60° C.,preferably 25-55° C., preferably 35-50° C., more preferably about 40° C.

Once the high resolution transmittance spectrum is obtained, the methodfurther involves measuring the column density CL of nitrogen dioxide inthe gaseous mixture using the high resolution transmittance spectrum.

The term “column density” as used here refers to a quantity thatmeasures the number of molecules per unit area projected along aparticular line of sight, e.g., a beam path of a laser. In view of that,the term “column density of nitrogen dioxide” as used here refers thenumber of molecules along a beam path of a laser. Accordingly, thecolumn density of nitrogen dioxide is an integrating a volumetricdensity of nitrogen dioxide p over the beam path length s of themultimode laser beam as shown in the following equation:

CL=∫ρ·ds  (8)

In the case of measuring the concertation of NO₂ in the gaseous mixture,e.g. air, the volumetric density of nitrogen dioxide is generallyconstant over the beam path length of the signal beam. Therefore, thecolumn density CL of nitrogen dioxide is a multiplication of the NO₂concertation in the gaseous mixture (e.g. in ppm) and the beam pathlength L (e.g. in meter).

According to the present disclosure, the column density CL of nitrogendioxide may be measured using Eq. 3, preferably using Eq. 4, whiletaking CL as a fitting parameter. In a preferred embodiment, the columndensity CL of nitrogen dioxide may preferably be measured using Eq. 4,wherein T(i) is the high resolution transmittance spectrum as a functionof the injection current, and the ratio of integrals in Eq. 4 may beobtained experimentally by measuring the transmittance of a well-knowncolumn density or by measuring integrals of the multimode laser beam ata known cross section, a, which may be obtained from literature atdifferent injection currents, as mentioned previously.

In a preferred embodiment, a detection limit for the column density ofnitrogen dioxide in the gaseous mixture, when measured by the method ofthe present disclosure, ranges from 0.1-1.0 ppm.m, preferably 0.2-0.8ppm.m, preferably 0.3-0.7 ppm.m, preferably 0.4-0.6 ppm.m, morepreferably about 0.5 ppm.m.

Once the column density is measured, the method further involvescalculating the concentration of nitrogen dioxide in the gaseous mixturefrom the column density. The concentration of nitrogen dioxide in thegaseous mixture may preferably be calculated by dividing the columndensity CL by the beam path length L of the signal beam.

According to the abovementioned detection limit ranges for the columndensity of nitrogen dioxide, in some preferred embodiments, a detectionlimit for the concentration of nitrogen dioxide in the gaseous mixtureranges from 0.5-10 ppb, preferably 0.6-5 ppb, preferably 0.8-2 ppb,preferably about 1 ppb, when the signal beam travels through the gaseousmixture with a beam path length of 100-600 m, preferably 300-500 m. Yetin some preferred embodiments, a detection limit for the concentrationof nitrogen dioxide in the gaseous mixture ranges from 0.1-10 ppm,preferably 0.5-5 ppm, preferably 0.8-2 ppm, preferably about 1 ppm, whenthe signal beam travels through the gaseous mixture with a beam pathlength of 0.1-2.0 m, preferably 0.2-1.0 m, preferably about 0.5 m.

In terms of the present disclosure, the term “detection limit” refers tothe smallest detectable value of a measurable physical quantity, e.g.,the column density CL, the concentration of nitrogen dioxide, etc.

In one embodiment, the gaseous mixture is disposed in an elongatedcompartment with a length of 0.01-10 m, preferably 0.1-5 m, preferably0.5-2 m, wherein at least a portion of the elongated compartment istransparent to a UV light and/or a visible light, thus allowing the UVlight and/or the visible light to pass therethrough. According to thisembodiment, the reference medium may be disposed in a second compartmentthat is substantially the same as the elongated compartment. In somepreferred embodiments, the gaseous mixture is first disposed in theelongated compartment and the first voltage signal is recorded, then thereference medium is disposed in the elongated compartment and the secondvoltage signal is recorded.

In some preferred embodiments, the gaseous mixture is maintained in theelongated compartment at a pressure of 0.5-1.5 atm, preferably 0.8-1.2atm, preferably about 1.0 atm. In certain embodiments, the gaseousmixture may be maintained in the elongated compartment at a pressureabove 1.5 atm, but preferably no more than 5 atm, preferably no morethan 4 atm. In some embodiments, the gaseous mixture is maintained inthe elongated compartment at a temperature of 10-70° C., preferably20-60° C., preferably 30-50° C. The gaseous mixture may have atemperature above 70° C. In these embodiments, the reference medium maypreferably be maintained at substantially the same temperatures andpressures.

In some preferred embodiment, the gaseous mixture is delivered to theelongated compartment in a continuous fashion at a volumetric flow rateof 50-200 ml/min, preferably 80-150 ml/min, preferably about 100 ml/min.Various equipment as known to those skilled in the art may be utilizedto maintain the pressure of the gaseous mixture in the range of 0.5-1.5atm, preferably 0.8-1.2 atm, preferably about 1.0 atm. Exemplaryequipment may include, without limitation, a vacuum pump such as aroughing pump, a valve such as a needle valve, a pressure gauge such asan absolute capacitance manometer, etc.

In one embodiment, the tunable laser diode and the beam splitter may belocated outside of the elongated compartment, wherein the signal beammay be transmitted through a transparent window located on one end ofthe elongated compartment. As defined herein, “transparent” refers to anoptical quality of a material wherein a certain wavelength or range ofwavelengths of light may traverse therethrough with a small loss oflight intensity. Here, the “transparent window” may causes a loss ofless than 5%, preferably less than 2%, more preferably less than 1% ofthe intensity of the signal beam.

Employing several absorption features (e.g. rotational-vibrationalfeatures) using the method of the present disclosure may providedetection of trace molecular species other than nitrogen dioxide, e.g.,carbon dioxide, carbon monoxide, nitrous oxide, methane, explosivecompounds, steroids, drugs, etc. The detection limit of NO₂concentration using the method of the present disclosure issubstantially smaller, for example an order of magnitude smaller, thanthe detection limit of NO₂ concentration when measured usingconventional methods such as, for example, direct or wave-modulatedabsorption spectroscopy. Moreover, the method of the present disclosureoffers a shorter data acquisition time period, for example at least 50%shorter, for real-time monitoring of NO₂ when compared to conventionalmethods. Also, since the method of the present disclosure is lessexpensive due to the absence of expensive laser diodes, e.g. externalcavity laser diodes. In view of that, the method of the presentdisclosure may be applied to analyzers that are utilized in industrialapplications for detecting and measuring concentrations of gaseousmolecules such as nitrogen dioxide. The method may also be used inmonitoring pollutants, e.g. NO₂, in the atmosphere.

Referring to FIG. 1, according to a second aspect, the presentdisclosure relates to a system 100 for measuring the concentration ofnitrogen dioxide in a gaseous mixture. The system 100 includes at leasta tunable laser diode 102, a laser diode current controller 103, a laserdiode temperature controller 105, a beam splitter 106, a first detector110 and a second detector 114, an elongated compartment 124, a vacuumpump 136 that is fluidly connected to the elongated compartment 124,electronic couplings and gas tubing interconnections, valves, flowcontrollers, sensors for monitoring temperature, pressure and gas flowrates, and a computer 122. The system 100 may optionally include opticalcomponents such as spectrometers, optically non-linear crystals, opticalfilters, neutral density filters, external optical resonators, beamcollimators, etc. In one embodiment, one or more of the tunable laserdiode 102, the laser diode current controller 103, the laser diodetemperature controller 105, the beam splitter 106, and theabovementioned optical components, when present, are arranged in athermally insulated enclosure 120. The thermally insulated enclosure 120may comprise a box fabricated from, for instance, various sheet panelsof a thermal insulating material, which provides atemperature-controlled environment for various optical components housedtherein. In some embodiments, several temperature sensors (not shown inFIG. 1) may be provided in the thermally insulated enclosure formonitoring an internal temperature of the thermally insulated enclosure.Said temperature sensors may transmit feedback signals to the computerthat controls and stabilizes the internal temperature of the thermallyinsulated enclosure. The thermally insulated enclosure may include oneor more heating (or cooling) devices which respond to signals providedby the computer.

In one embodiment, the elongated compartment 124 has a first end and asecond end separated by a side wall along a longitudinal axis of theelongated compartment, wherein at least a portion of the first end andat least a portion of the second end is transparent to a UV light and/ora visible light in a wavelength range of 250-650 nm, preferably 350-500nm. For example, in one embodiment, a transparent window may be presentin the first end and the second end, wherein the transparent windows maycomprise quartz, glass, or a polymeric material transparent to UV/Vislight such as polymethylmethacrylate (or PLEXIGLAS). In a preferredembodiment, the transparent windows at the first end and the second endmay be tilted with an angle of 30-60°, preferably about 45° relative tothe longitudinal axis of the elongated compartment, as shown in FIG. 1.

The elongated compartment 124 may have a rectangular shape, preferably acylindrical shape, or preferably a trapezoidal shape, as shown inFIG. 1. A length of the elongated compartment 124 may vary in the rangeof 0.01-10 m, preferably 0.1-5 m, preferably 0.5-2 m, and a volume ofthe elongated compartment 124 may be no more than 10 m³, preferably nomore than 1.0 m³, preferably in the range of 1-1000 L (liter),preferably 10-500 L.

In some embodiments, the elongated compartment 124 includes at least onesealable aperture arranged thereon for delivering the gaseous mixture orthe reference medium to the elongated compartment and/or discharging thegaseous mixture or the reference medium from the elongated compartment.Accordingly, in one embodiment, the elongated compartment 124 includesone sealable aperture that can be opened and closed, for example, with avalve connected thereto, wherein the sealable aperture is opened duringflowing the gaseous mixture or the reference medium to the elongatedcompartment 124 and is closed during passing the signal beam 112 throughthe elongated compartment 124. As shown in FIG. 1, in some preferredembodiments, the elongated compartment 124 includes two sealableapertures arranged thereon with two valves for opening and closing eachsealable aperture, wherein a first sealable aperture (i.e. an inlet) isconfigured for delivering the gaseous mixture or the reference medium tothe elongated compartment 124 and a second sealable aperture (i.e. anoutlet) is configured for discharging the gaseous mixture or thereference medium from the elongated compartment 124. A gas flowcontroller 130 and several sensors (not shown in FIG. 1) may beinstalled on the elongated compartment 124 for real-time monitoring ofthe temperature, the pressure, and/or the flow rate of the gaseousmixture or the reference medium inside the elongated compartment 124.The gaseous mixture may be stored in a first tank, e.g. tank 126,whereas the reference medium may be stored in a second tank, e.g. tank128. Appropriate utilities such as pipelines, valves, flowratecontrollers, etc. as known to those skilled in the art may be employedfor selective delivery of the gaseous mixture or the reference medium tothe elongated compartment. In some embodiments, a vacuum pump 136 suchas a roughing pump, and a needle valve 134 are used for maintaining thegaseous mixture or the reference medium at a preferred pressure rangeinside the elongated compartment 124. A pressure gauge 132 may be usedfor real-time monitoring the pressure of the gaseous mixture or thereference medium inside the elongated compartment 124.

The system 100 further includes a tunable laser diode 102, as describedpreviously, located at a distance from the elongated compartment 124 forgenerating a multimode laser beam 104 and emitting the multimode laserbeam 104 to the elongated compartment 124. In one embodiment, thetunable laser diode 102 is arranged such that the multimode laser beam104 is substantially parallel to the longitudinal axis of the elongatedcompartment 124, as shown in FIG. 1. The distance between the tunablelaser diode 102 and the elongated compartment 124 may be preferably nomore than 1 m, preferably 10-80 cm, preferably 20-50 cm. The beamsplitter 106, which is arranged in the distance between the tunablelaser diode 102 and the elongated compartment 124, preferably reflects afirst portion of the multimode laser beam 104, i.e. the reference beam108, downwardly to the second detector 114, whereas a second portion ofthe multimode laser beam 104, i.e. the signal beam 112, travels throughthe beam splitter 106 and enters the elongated compartment 124, as shownin FIG. 1. Upon leaving the elongated compartment 124, the signal beam112 may hit the first detector 110. In one embodiment, the systemincludes a secondary beam splitter 116 that splits a portion of themultimode laser beam 104 to the spectrometer 118 for visualizing and/orcalibrating the multimode laser beam 104.

The system 100 further includes a laser diode current controller 103 anda laser diode temperature controller 105, as described previously,wherein each is electrically connected to the tunable laser diode 102for varying the injection current and the diode temperature of thetunable laser diode 102. In one embodiment, the laser diode currentcontroller 103 is a commercially available LDC200C provided by Thorlabs,which is in communication with the computer 122 preferably with aLabView and a 16-bit interface card from National Instrument modelUSB-6251. In another embodiment, the laser diode temperature controller105 is a commercially available TED200C provided by Thorlabs, which isin communication with the computer 122 preferably with a LabView and a16-bit interface card from National Instrument model USB-6251.

The system 100 further includes the first detector 110 for detecting thesignal beam 112 after being passed through the gaseous mixture insidethe elongated compartment 124. In addition, the system 100 furtherincludes the second detector 114 for detecting the reference beam 108after being passed through the reference medium. The type of the firstand the second detector s are not meant to be limiting and variousdetectors, as described previously, may be utilized here. For example,in a preferred embodiment, the first and the second detectors aresubstantially the same, wherein each is a silicon photodiode detector,e.g. Thorlabs model PDB210A, which is in communication with the computer122. Each of the first and the second detector may be provided as apackage that includes an internal thermoelectric cooler, an internaltemperature sensor such as a thermistor, a signal amplifier such as atrans-impedance amplifier that converts current into a proportionalvoltage, a digitizer, etc.

The system 100 further includes the computer 122 that is connected tothe first detector 110 and the second detector 114, wherein the computer122 receives a first voltage signal from the first detector 110 and asecond voltage signal from the second detector 114 and calculates theconcentration of nitrogen dioxide in the gaseous mixture. The computer122 may be used for one or more of controlling the injection current andthe diode temperature of the tunable laser diode, calibrating themultimode laser beam, controlling and monitoring the flow rate, thetemperature, and the pressure of the gaseous mixture or the referencemedium in the elongated compartment, controlling and monitoring thetemperature inside the thermally insulating enclosure, etc.

In some embodiments, the computer 122 may include one or more componentsof a personal or desk computer including a motherboard with amicroprocessor, cache memory, ROM memory, random access memory such as ahard disk, optical disk drive or flash memory, an operating system,control software, various external communication ports at its front andrear panels for sending information to and receiving information fromthe computer. Example of such external communication ports may includeUniversal Serial Bus (USB) ports for connecting a keyboard, an externalmemory device, etc. to the computer; an Ethernet port for connecting thecomputer to an external network or to other computers and a VGA/HDMIport for providing detailed visual information display to a user.

The examples below are intended to further illustrate protocols for themethod of measuring the concentration of NO₂ in a gaseous mixture, andare not intended to limit the scope of the claims.

Example 1

The following examples illustrate a method to identify and measure theabsorption of NO₂ over a wavelength range of few nanometers using atunable Fabry-Perot multimode blue laser diode.

According to the Lambert-Beer's law, when a light of intensity I₀(λ)passes through a sample of length L with molecular number density C andmolecular cross section σ(λ), the intensity of transmitted lightI_(t)(λ) may be measured by the following equation:

I _(t)(λ)=I ₀(λ)e ^(−σ(λ)CL)  (1)

where the incident light, the transmitted light and the cross sectionare all functions of the light wavelength λ. If the light is directlydetected by an optical detector, such as a photodiode, the signalregistered by the detector is proportional to the intensity integratedover the wavelength range of the light. Hence, if the transmittance ismeasured by two detectors, one measuring the incident beam and the othermeasuring the transmitted beam, the measured transmittance T is given bythe following equation:

$\begin{matrix}{T = \frac{\int_{\lambda\; s}^{\lambda_{e_{{I_{0}{(\lambda)}}e^{{- {\sigma{(\lambda)}}}{CL}_{d\;\lambda}}}}}}{\int_{\lambda_{s}}^{\lambda_{e_{{I_{0}{(\lambda)}}d\;\lambda}}}}} & (2)\end{matrix}$

where λ_(s) and λ_(e) are the start and end of the spectral range of theincident light, respectively. Here, we assume the detector efficienciesare constant as we will use small tuning ranges of about 2 nm. If thelight source has a spectral width smaller than some features of thecross section σ(λ), and if the light source can be tuned over thesefeatures, then one can obtain a varying transmittance as a function ofthe tuning parameters of the light source. This varying transmittanceprovides a unique signature of the investigated molecule.

The X²A₁→A²B₂ visible absorption band of NO₂ extends from about 300 to600 nm peaking around 400 nm with a relatively large cross section ofabout 6×10⁻¹⁹ cm². Although, individual spectral lines of this band arenot resolvable at ambient atmospheric pressure due to collisionalbroadening, there are many relatively sharp features some of which witha relative variation as large as 60% and a full-width-half-maximum ofabout one nm, such as that around 448 nm which we will use.

A typical laser diode can be used to obtain significant variations inabsorption as it is tuned over a specific feature of the NO₂ visibleband. A typical blue laser diode, operating well above threshold, hasmany longitudinal modes, more than 20 modes, and its modes cover aspectral range of about one nm. The spectral range starts just abovethreshold with a fraction of one nm with one or two longitudinal modesand widens in range to about two nm as the injection current isincreased. In addition, the range also shifts toward higher wavelengthwith the injection current. The spacing between two adjacentlongitudinal modes is typically 0.05 nm and is given by Δλ=λ²/2 nL,where λ is the wavelength, n≅3 is the index of refraction of the gainmedium of the laser, gallium nitride, and L is the length of the lasercavity typically 600 μm. The laser modes can be tuned rapidly by varyingthe injection current or can be tuned at slower rate by altering thetemperature. Typically, a mode can by tuned at rate of about 0.005 nm/mAand 0.015 nm/° C. toward longer wavelengths. This corresponds to atuning range of a specific mode about 0.6 nm for the allowed maximuminjection current range of 130 mA and a tuning range of about one nm forthe allowed maximum range of temperature of 60° C. See for example[Romadhon, M. S., Aljalal, A., Al-Basheer, W., Gasmi, K., “Longitudinalmodes evolution of a GaN-based blue laser diode,” Opt. Laser Technol.70, 59-62, Elsevier Ltd (2015)—incorporated herein by reference in itsentirety].

The temperature of laser diode can be adjusted so that the tuning rangeby current produces the most pronounced variation in the transmittance.Hence, the transmittance T is a function of the laser current, as wellas the laser spectrum, the start and the end of the laser spectrumrange, as shown in the following equation:

$\begin{matrix}{{T(i)} = {\frac{\int_{\lambda_{s}{(i)}}^{\lambda_{e^{(i)}{I_{0}{({\lambda,i})}}e} - {{\sigma{(\lambda)}}{CL}_{d\;\lambda}}}}{\int_{\lambda_{s}{(i)}}^{{\lambda_{e}{(i)}}_{{I_{0}{({\lambda,i})}}{d\lambda}}}}.}} & (3)\end{matrix}$

For very small absorptions, as in the case of monitoring NO₂ in ambientair, equation (3) can be approximated by the following equation:

$\begin{matrix}{{T(i)} \cong {1 - {{CL}{\frac{\int_{\lambda_{s}{(i)}}^{{\lambda_{e}{(i)}}_{{I_{0}{({\lambda,i})}}{\sigma{(\lambda)}}{d\lambda}}}}{\int_{\lambda_{s}{(i)}}^{{\lambda_{e}{(i)}}_{{I_{0}{({\lambda,i})}}d\;\lambda}}}.}}}} & (4)\end{matrix}$

Thus, the column density CL can be measured by fitting the measuredtransmittance to the right-hand side of either Eq. 3 or Eq. 4 with CL asthe fitting parameter. The ratio of integrals in Eq. 4 was obtainedexperimentally by measuring the transmittance of a well-known columndensity. The ratio of integrals can alternatively be obtained byintegrating the spectra of the laser that is obtained at differentinjection currents, wherein the cross section can be obtained fromliterature.

The shape of the transmittance curve as function of the laser currentdepends on the laser diode temperature. In order to enhance thesignal-to-noise ratio, the temperature was set to a preferredtemperature (e.g. 40° C. or 50° C.) to obtain a curve with the biggestvariation. A detection limit for column density of about 0.5 ppm.m wasachieved with short integration time, which corresponds to a detectionlimit of about 1 ppm for a length of 50 cm.

Example 2

FIG. 1 shows a schematic diagram of the experimental setup. A blue laserdiode Roithner LaserTechnik GmbH model LD-445-50PD is utilized, whichhas a nominal output wavelength of 445 nm with an output power of up to50 mW. The laser diode was housed in a laser diode mount model TCLDM9from Thorlabs. The injection current and the diode temperature werecontrolled using Thorlabs LDC200C and TED200C controllers, respectively.Both controllers were connected to a PC using a LabView VI and a 16-bitinterface card from National Instrument model USB-6251.

To measure the spectrum of the laser, part of the laser beam wasdirected to a spectrometer that includes a SPEX 500M monochromator and aThorlabs LC1-USB line camera. The monochromator has a resolution of 0.02nm and the wavelength of the monochromator is calibrated with sevenspectral lines from a Kr discharge lamp. The laser spectra are collectedat injection currents ranging from 30 mA to 160 mA in a step of 10 mA.This measurement has to be done only once at a specific lasertemperature.

Before entering the elongated compartment, part of the laser beam wasdirected to a silicon photodiode detector from Thorlabs model PDB 210A.This beam was used to normalize the laser intensity variation and thusis called the reference beam. The rest of the laser beam, i.e. thesignal beam, was passed through the elongated compartment and furtherdetected by another silicon photodiode detector model PDB 210A. Bothdetectors were connected to the PC with a USB-6251 interface. Neutraldensity filters were also used to attenuate the laser beam intensities.For the transmittance measurements, the laser temperature was fixed, andreadings from both detectors were recorded at each step of the laserinjection current in the range from 30 mA to 160 mA in a step of 0.1 mA.

The elongated compartment has a length of 40 cm and was made fromstainless steel. The elongated compartment windows were tilted toeliminate interferences due to the windows' surfaces. A NO₂/N₂ gasmixture with an NO₂ concentration of 200 ppm was flowed into theelongated compartment in a controlled flow rate using a mass flowcontroller, Omega FMA5412A-ST. Also a high purity N₂ gas was flowed intothe elongated compartment in a controlled flow rate using a mass flowcontroller, Omega FMA5412A-ST. The gas flow rate was controlled toreduce the effect of NO₂ adsorption on the surface of the elongatedcompartment. The pressure in the elongated compartment was maintained atatmospheric pressure using a roughing pump, a needle valve, and anabsolute capacitance manometer model MKS 690A13TRA.

Example 3

FIG. 2 shows the absorption cross section of NO₂ molecules [Yoshino, K.,Esmond, J. R., Parkinson, W. H., “High-resolution absorption crosssection measurements of NO₂ in the UV and visible region,” Chem. Phys.221(2), 169-174 (1997)], and three spectra of the laser beam at a lasertemperature of 40° C. and a wavelength region that the laser used herecan be tuned. The laser tuning range is from about 447.2 nm to 449.2 mm,which covers one of the most pronounced features of NO₂ cross section at448 nm. The fractional variation ((σ_(max)−σ_(min))/σ_(g)) of thisfeature is about 0.60 with full-width-half-maximum of 0.8 nm. FIG. 2shows the three spectra of the laser, one at an injection current of 30mA just at the laser threshold, one at an injection current of 100 mA,and the last one at an injection current of 160 mA, which corresponds tothe maximum allowable injection current. As the injection currentincreases, the laser spectrum becomes wider and shifts toward higherwavelengths.

The continuous curve in FIG. 3 is the measured transmittance of theNO₂/N₂ gas mixture with the NO₂ concentration of 200 ppm. To obtain thetransmittance, two separate measurements were used, one fornormalization, when pure N₂ gas was flowed into the elongatedcompartment, and the other when the NO₂/N₂ gas mixture was flowed intothe elongated compartment. Hence, the measured transmittance T is givenby the following equation:

$\begin{matrix}{T = \frac{\left( {V_{sig}/V_{ref}} \right)_{{NO}_{2}/N_{2}}}{\left( {V_{sig}/V_{ref}} \right)_{N_{2}}}} & (5)\end{matrix}$

where V_(sig) and V_(ref) are the voltages of the signal detector andthe reference detector, respectively. FIG. 3 also shows the calculatedtransmittance obtained from the cross section of NO₂ and measured laserspectra using Eq. 3 with a column density of CL 200 ppm×40 cm, inaccordance with our experiment parameters.

Although the measured and calculated transmittance spectra revealed thesame curvature with a minimum value at a laser injection current ofabout 100 mA, there is a difference between the two transmittancespectra. The difference may happen due to the adsorption of a portion ofNO₂ molecules on the surface of the elongated compartment. In thesemeasurements, a flow rate of 100 ml/min was utilized. Another probablereason for the mismatch between the measured and calculatedtransmittance spectra may be due to the inaccuracy of NO₂ concentrationin the NO₂/N₂ gas mixture as claimed by the vendor. However, theinsignificant mismatch does not reject the notion that a regularmultimode blue laser diode can be used to identify NO₂ by providingtransmittance over a range of wavelengths and, hence, reducing thechance of misidentifying absorption from NO₂ with absorption from otherbroadband interferers such dust particles.

From the range of variation and the noise level in the experimentaltransmittance, a detection limit of 2 ppm over a length of 40 cm isestimated. This detection limit corresponds to a detection limit ofabout 2 ppb over a distance of 400 m, which makes the method of thepresent disclosure suitable for detecting NO₂ pollution over long paths.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, defines, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

1.-18. (canceled)
 19. A blue laser system for measuring nitrogen dioxideconcentration in a gaseous mixture, the system comprising: an elongatedcompartment with a first end and a second end separated by a side wallalong a longitudinal axis of the elongated compartment, wherein at leasta portion of the first end and at least a portion of the second end eachhave transparent windows transparent to a UV light and/or a visiblelight and having a light intensity loss of less than 1%, wherein thetransparent windows are tilted with an angle of 30-60° relative to thelongitudinal axis; at least one sealable aperture arranged on theelongated compartment for delivering a gaseous mixture to the elongatedcompartment and/or discharging the gaseous mixture from the elongatedcompartment; a tunable blue laser diode having a nominal outputwavelength of 445-455 nm with an output power of up to 50 mW arranged ata distance from the elongated compartment for generating a multimodelaser beam and emitting the multimode laser beam to the elongatedcompartment, wherein the tunable laser diode is arranged such that themultimode laser beam is substantially parallel to the longitudinal axisof the elongated compartment; a laser diode current controller that iselectrically connected to the tunable laser diode for varying aninjection current of the tunable laser diode; a laser diode temperaturecontroller that is electrically connected to the tunable laser diode forvarying a diode temperature of the tunable laser diode; a beam splitterarranged between the tunable laser diode and the elongated compartmentfor splitting the multimode laser beam into a signal beam and areference beam, wherein the signal beam is passed through the gaseousmixture inside the elongated compartment and the reference beam ispassed through a reference medium, wherein the beam splitter is selectedfrom the group consisting of a dielectric mirror, a beam splitter cube,a fiber-optic beam splitter and a waveguide beam splitter, and isoptically coupled to the multimode laser beam at an incidence angle ofabout 45°; a first detector for detecting the signal beam after beingpassed through the gaseous mixture inside the elongated compartment; asecond detector for detecting the reference beam after being passedthrough the reference medium; and a computer connected to the first andthe second detectors, wherein the computer receives a first voltagesignal from the first detector and a second voltage signal from thesecond detector and calculates a concentration of nitrogen dioxide inthe gaseous mixture.
 20. (canceled)